Engadget’s Chris Velazco got to sit down with Phil Schiller to talk about the iPhone XR:

To add to the curiosity of it all, the R doesn’t mean much either.
Phil Schiller, gingerly gripping a cup of coffee across from me,
said the letters Apple uses never stand for something specific.
But then his voice softened a little as he started to tell me
about what the letters mean to him.

“I love cars and things that go fast, and R and S are both letters
used to denote sport cars that are really extra special,” he said
with a smile.

It just isn’t worth worrying whether the “R” (or “S” for that matter) stands for anything in particular. R sounds cool and is one click “less than” S.